1. Field of the Invention
This invention relates generally to resist layer structures and more particularly to high sensitivity resist layers useful in lithographic processes.
Optical or electron beam lithography is still used as the main tool in micro-circuit fabrication. It is expected to continue into the near future.
The additive metallization technique known as "lift-off" was developed with the advent of electron beam lithography in the late 1960's. It provides the metallization after the exposure and development of the resist. Basically, the lift-off technique utilizes the fact that electron scattering in the resist and back scattering from the substrate creates a tear-shaped energy absorption profile in the resist, which results in an undercut profile after resist development. In this way metal which is evaporated over the entire surface exhibits discontinuities between the metal on the substrate and the metal over the resist. During resist removal in a suitable solvent the metal over the resist is also removed and a clean and faithful reproduction of the image is obtained in metal. An additional advantage of the lift-off technique is that multi-level metal structures can be formed because any material or combination of materials that can be evaporated can be used.
One of the principal reasons for the success of the lift-off process in electron beam lithography is the fact that the energy absorption in the resist film during exposure is not linear but reaches a maximum in about two-thirds of the beam penetration range. Thus, with proper exposure and development, undercut resist profiles are easily obtainable.
In the optical exposure of photoresist, however, energy absorption is highest at the top of the resist film and lowest at the interface between the resist and the substrate due to the attenuation of the light in the resist. Moreover, standing waves created by the light reflected from the substrate are a further complication. These exposure conditions make it impossible to obtain undercut or even vertical resist profiles with normal UV exposure of AZ type positive photoresist.
Another technique which has been successfully in electron beam lithography to increase resist sensitivity, while maintaining the undercut feature of the developed resist, comprises the coating of two or more resist layers having widely different solubilities. After electron beam exposure, a developer is chosen which develops the top layer at least ten times slower than the bottom layer. Alternatively, two mutually exclusive developers can be used for the successive development of the two layers. Both of these approaches result in resist profiles suitable for lift-off metallization.
With resists which can be used with both electron beam as well as optical exposure systems, however, it is difficult to spin-coat two distinct layers without excessive interaction at the interface, due to the low prebake temperature allowed for these resists between coatings. Electron resists such as polymethylmethacrylate (PMMA) can be baked at up to 170.degree. without deterioration; however, an AZ-type resist which is suitable for both optical and electron beam exposure cannot be baked above about 100.degree. C. At this low temperature the resist film retains its solubility in various solvents and can therefore be easily dissolved by the application of the second layer.
Therefore, based on results with E-beam type resists, it has been well known that the application of two layers of standard photoresist, of which the first layer is more soluble in a developer, could be used to form a relief mask with recessed sidewalls for a lift-off process. This type of structure could be made in several ways: (1) by reducing the sensitizer concentration in the lower part; (2) by use of a resin having a higher molecular weight than the lower part of the resist structure which develops faster than the upper portion; or (3) by use of a sensitizer in the lower part of the structure which is less resistant to the developer in the case of positive resist.
However, as previously mentioned, if the second layer of an optical resist is directly applied on the first layer prior to exposure, the layers dissolve and mix with each other. If the first layer is baked at high temperature to avoid this dissolution, the first layer hardens and becomes insoluble in the developer.
One successful lift-off process using standard optical-type resists is described in the patent issued in the names of Franco et al, U.S. Pat. No. 4,004,044 and entitled "Method for Forming Pattern Films Utilizing a Transparent Lift-Off Mask". The method comprises depositing an organic polymer masking layer, such as AZ-1350 type resist, atop a substrate. After baking to improve adhesion and thermal stability, a separate glass resin layer is spun on over the resist. A second masking layer which may also be AZ-1350 is then spun on the resist layer. The second masking layer is patterned and windows are opened in the glass layer and the first masking layer by reactive sputter etching using two gases. The second reactive sputter step is continued until the edges in the openings through the glass material overhang the edges in the openings through the first AZ-1350 layer, thereby forming a lift-off mask. Although the Franco et al technique is very effective in providing usable lift-off masks, it is complicated by the need for the glass resin layer, reactive sputter etching and relatively high temperature. A simpler process is desirable.